1. Field of the Invention
The present invention relates to a piezoelectric speaker for use in, for example, audio equipment, a method for producing the same, and a speaker system including such a piezoelectric speaker.
2. Description of the Related Art
An audio reproduction mechanism of a piezoelectric speaker is based on planar resonance. Conventional piezoelectric speakers have a structure in which a peripheral portion of a vibrating plate is fixed to a frame. In such a structure, the amplitude of the vibrating plate is significantly reduced toward the peripheral portion of the vibrating plate. As a result, the vibration energy which can be transmitted to the air from the peripheral portion of the vibrating plate is significantly reduced. Such a vibrating plate characteristic is the same as that of the vibration surface of a percussion drum.
For this reason, the conventional piezoelectric speakers have a problem in that a high sound pressure level is obtained in a high frequency range in which sound is reproduced at a relatively small amplitude, whereas a sufficiently high sound pressure level is not obtained in a low frequency range of about 1 kHz or less.
Accordingly, the conventional piezoelectric speaker are only applied, for example, for a tweeter for reproducing sound in a high frequency range and for a receiver of a telephone.
FIG. 22 shows a structure of a conventional piezoelectric speaker 220 including a vibrating plate sandwiched by a resin foam body. The piezoelectric speaker 220 includes a metal vibrating plate 224, a piezoelectric element 223 provided on the metal vibrating plate 224, and a resin foam body 222 for securing a peripheral portion of the metal vibrating plate 224.
The resin foam body 222 has flexibility and is provided so as to hold the metal vibrating plate 224.
The resin foam body 222 provided for increasing the amplitude of the metal vibrating plate 224 also has a contradicting role as a supporting element for securing the peripheral portion of the metal vibrating plate 224. In actuality, the resin foam body 222 is often provided more for securing the peripheral portion of the metal vibrating plate 224 rather than for increasing the amplitude of the metal vibrating plate 224. Accordingly, a sufficient compliance is not obtained.
The vibrating plate 224 of the piezoelectric speaker 220 behaves in a similar manner as that of the vibration surface of a percussion drum, and thus has difficulty in reproducing the sound in a low frequency range as in a conventional piezoelectric speaker in which a peripheral portion of a vibrating plate is fixed to a frame.
The piezoelectric speaker 220 also has an inconvenience that the thickness thereof, which is inevitably increased by the thickness of the resin foam body 222 and a frame (not shown) for holding the resin foam body 222, cannot be reduced to less than a certain level.
As described above, the conventional piezoelectric speakers have a problem of having difficulty in reproducing sound in a low frequency range. The conventional piezoelectric speakers have another problem that since a strong resonance mode is generated in a specific frequency, a large peak dip appears in the acoustic characteristics in a wide frequency range.